In horizontal continuous casting, a casting process in which a cast matter pulling cycle is composed of a pull step, a pause step and a push back step is known (Japanese Unexamined Patent Publication No. Sho-58-44950). FIG. 2 typically shows a pattern of the pulling velocity in the above steps.
In a horizontal continuous casting process for casting a cast matter having a cross sectional size of 80-350 mm at a high pulling velocity (not lower than 1.6 m/min), the pulling cycle is set to about 120 cycle/min, and time t.sub.o of one pulling cycle is set to about 0.5 sec. The respective times t.sub.1, t.sub.2 and t.sub.3 of the pull, pause and push back steps are set to 0.2 sec, 0.1 sec and 0.2 sec respectively. In this case, the pulling velocity V.sub.c in the pull step rises abruptly following an almost linear speed gradient in about 0.04 sec. That is, a cast matter is pulled suddenly with an almost linear velocity gradient k (=tan .theta.) from a start point A to a point B. Then, the cast matter is pulled at a uniform speed from the point B to a point C, and the velocity is decreased suddenly from the point C to a point D. Then the pulling is paused from the point D to a point E, and next the cast matter is pushed in the reverse direction, that is, back to the mold slightly from the point E. Then, returning to the start point A through points E, G and H, one pulling cycle is finished.
In such a conventional withdrawal control process, there is a particular problem in that outside air enters into a mold in the velocity-up stage in the step of pulling a cast matter. This phenomenon causes residual bubbles 2 in a surface layer portion of a cast matter 1 as shown in FIG. 3, and if the number of residual bubbles increases, the bubbles appear as linear flaws in the surface of products at the time of rolling so that the quality deteriorated. The cause of such residual bubbles is that the pull velocity in the velocity-up stage is so high that negative pressure is produced in a portion called a triple point 5 between a mold 3 and a brake ring 4 to lead the outside air therein, the air being brought into molten metal to be trapped as bubbles surface layer portion of of shell solidifying thereon.
In order to solve the problem of residual bubbles in a surface layer portion of a cast matter, a device of preventing the outside air from entering into the above-mentioned triple point has been made (Japanese Unexamined Utility Model Publication No. Hei-1-30687). A seal mechanism disclosed in this application is constituted by three members, that is, a mold 3, a brake ring 4 and a feed tube 7 which are joined with each other with a flexible thin plate 8 (carbon sheet or the like) inserted as a gasket into a joint portion of the three members.
However, the gasket system is not always reliable and requires skilled workers and rather long fitting work time. Accordingly, the gasket system is not recommendable.
The present invention is intended to prevent the outside air from entering into a mold only through pulling acceleration control in the consideration of the foregoing disadvantage and inconvenience caused by employing such a mechanical seal mechanism, and it is an object of the present invention to provide a withdrawal control process of horizontal continuous casting in which the number of bubbles in a surface layer portion of a cast matter can be reduced extremely.